In a local distribution network, the medium for the transmission of information between the various pieces of equipment is of a type which enables each piece of equipment to discern the transmitted messages. The physical means used to generate the elementary signals of the messages can be any whatsoever. The message can for example be defined in electromagnetic form, i.e. carried by electrons, or in optical form, i.e. carried by photons.
As for the media of a distributed communication, they can be passive, semi-passive or active. They are called passive when they do not include any units having the function of regenerating or amplifying the signals of the message. (Such units naturally require the use of an external power source). The media are called semi-active when they contain such regenerating or amplifying units. Finally, active media include units whose function is to modify all or part of the signals of the message during their passage. The present invention applies to these different categories of distribution media, noting that the active media mentioned last can in certain cases have application peculiarities.
The automated pieces of equipment that are going to be interconnected by the network can be any whatsoever. They can for example be sensors/activators, programmable robots, controllers/regulators, data processing terminals, computers, telephonic pieces of equipment or video terminals.
These equipments are connected to access stations which manage the communications on the shared medium or media. Each station operates independently and asynchronously with respect to the other stations. Each station is physically connected to the distribution medium that it shares with the other stations, by means of hardware and software assembly called a coupler.
Thus the function of a coupler is to provide the generation and reception of the physical signals on the communications medium, and to carry out an access protocol managing the sharing of that medium and the transmission and reception of information structured in the form of messages.
The present invention relates more particularly to the functioning of these couplers.
There are as many couplers as there are stations connected to the local network. In principle, they are all identical. The only common resource shared by the stations is therefore the physical communications medium. The couplers operate asynchronously and cannot instantaneously know what the other couplers are doing at any time. Such a system gives rise to the possibility of an attempt to simultaneously transmit different messages originating from several couplers. Such an attempt at simultaneous transmission causes what is called a collision.